Existing mass spectrometers have an ion source that produces ions of a sample material. These ions are then processed by a mass analyzer which includes a mass detector. Some existing ion sources produce ions using a technique known as electron ionization (EI). Particles of a sample material that are referred to as analytes are supplied in a gas phase to an ion volume having a relatively low pressure, and a stream of electrons is also supplied to the ion volume. The electrons directly strike the sample analytes, and the resulting energy exchange is sufficient to cause ionization, producing ions characteristic of the sample material. These ions are then supplied to the mass analyzer.
A different type of ion source produces ions using a technique known as chemical ionization (CI). The analytes of the sample material are supplied in a gas phase to an ion volume, and a reagent gas such as methane is also supplied to the ion volume. Further, a stream of electrons is supplied to the ion volume. The ion volume is configured so that the inflow of the reagent gas maintains a relatively high pressure within the ion volume, thereby ensuring a density for the reagent gas that increases the probability of collisions between the incoming electrons and the molecules of the reagent gas. When electrons collide with the molecules of the reagent gas, the collisions produce ions of the reagent gas. The ions of the reagent gas then react with the analytes of the sample gas, in order to form further ions that are characteristic of the sample material. These further ions are then supplied to the mass analyzer.
In both EI and CI, an electron source is configured to selectively provide the stream of electrons to the ion volume. The electron source includes a filament that is energized to emit electrons for the stream. It is advantageous to provide a second filament. When one of the filaments burns out, an operator can continue running samples with the other filament. As such, the mass spectrometer is not rendered completely inoperative by a burned-out filament, and can continue operating with minimum disruption.
In one approach, two separate filaments are provided with one filament on each side of the ion volume. While this approach has been generally adequate for its intended purposes, it has not been entirely satisfactory in all respects. As one example, this approach increases the cost associated with manufacturing the mass spectrometer. Moreover, the design of this approach is more complex due to the electrical and mechanical connections that are required on both sides of the ion volume. In another approach, two separate filaments are supported on common structure that is positioned on one side of the ion volume. The two filaments are spaced from each other in a direction transverse to a direction of electron travel to the ion volume.